ath10k: add board 2 API support

QCA6174 needs different board files based on board type. To make it easier to
distribute multiple board files and automatically choose correct board file
create a simple TLV file format following the same principles as with FW IEs.
The file is named board-2.bin and contain multiple board files. Each board file
then can have multiple names.

ath10k searches for file board-N.bin (where N is the interface version number
for the board file, just like we for firmware files) in /lib/firmware/*, for
example for qca99x0 it will try to find it here:

/lib/firmware/ath10k/QCA99X0/hw2.0/board-2.bin

If ath10k doesn't find board-2.bin then it will fallback to the old board.bin file.

This patch adds a simple name scheme using pci device id which for now will be
used by qca6174:

bus=%s,vendor=%04x,device=%04x,subsystem-vendor=%04x,subsystem-device=%04x

This removes the old method of having subsystem ids in ar->spec_board_id and
using that in the board file name.

Signed-off-by: Manikanta Pubbisetty <c_mpubbi@qti.qualcomm.com>
[kvalo@qca.qualcomm.com: simplified the file format, rewrote commit log, other smaller changes]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>